1. Field of the Invention
The present invention relates to a displacement measuring apparatus for measuring the displacement information of a moving object or fluid, and more particularly to a displacement measuring apparatus for measuring the information such as a velocity by detecting the deviation of frequency of a coherent light flux such as a laser beam.
2. Related Background Art
Conventionally, a laser Doppler velocimeter has been known as an apparatus for measuring the movement velocity of a moving object or fluid (thereinafter referred to simply as "moving object") in non-contact and at high accuracy. The laser Doppler velocimeter is an apparatus for measuring the movement velocity of a moving object by directing the radiation of a laser beam to the moving object and using an effect that the frequency of light scattered by the moving object deviates (or shifts) in proportion to the movement velocity, or a so-called Doppler effect.
One example of this conventional laser Doppler velocimeter is shown in FIG. 1. In the figure, 1 is a laser, 2 is a collimator lens, 3 is a parallel light flux, 4 is a beam splitter, 6 and 6' are reflecting mirrors, 7 is a moving object moving in the arrow direction at a velocity V, 8 is a focusing lens, and 9 is a photodetector.
In this constitution, a laser beam emitted from the laser 1 is made into the parallel light flux 3 through the collimator lens 2, and divided into two light fluxes 5 and 5' by the beam splitter 4. The two light fluxes 5, 5' are then reflected at the reflecting mirrors 6 and 6', respectively, and radiated onto the moving object 7 moving at a velocity V at an incident angle .theta., said two light fluxes overlapping. The light scattered by the moving object is detected via the focusing lens 8 by the photodetector 9. The frequencies of scattered lights from two light fluxes are subjected to the Doppler shifts of +.DELTA.f and -.DELTA.f, respectively, in proportion to the movement velocity V of the moving object. Here, the wavelength of laser beam is .lambda., .DELTA.f can be given by the following expression (1). EQU .DELTA.f=V sin (.theta.)/.lambda. (1)
The scattered lights having undergone the Doppler shifts of +.DELTA.f and -.DELTA.f, respectively, interfere with each other to cause a periodical variation of contrast on a light receiving plane of the photodetector 9. Its frequency F can be given by the following expression (2) EQU F=2.DELTA.f=2V sin (.theta.)/.lambda. (2)
Hence, the velocity V of the moving object 7 can be obtained by measuring the frequency F (thereinafter referred to as Doppler frequency) of the photodetector 9 from the expression (2).
With the laser Doppler velocimeter such as the conventional example as above described, the Doppler frequency F is inversely proportional to the wavelength .lambda. of the laser beam. Accordingly, it was necessary to use a laser light source having a stable wavelength as the light source of the laser Doppler velocimeter. As the laser light source capable of continuous emission and having a stable wavelength, a gas laser such as a He-Ne laser is often used, but the laser oscillator main body is large and requires a high voltage, resulting in a large and expensive apparatus. On the other hand, a laser diode (or semiconductor laser) for use in a compact disk, a video disk or in optical fiber communication is very small and easy to drive, but has a problem that the temperature dependence exists.
FIG. 2 shows one example of the normal temperature dependence of a laser diode (quote from Mitsubishi semiconductor data book, 1987: optical semiconductor devices), in which a portion with continuously changing wavelength is mainly due to the temperature variation of the refractive index in the active layer of the laser diode, and is 0.05 to 0.06 nm/.degree. C. On the other hand, a portion with discontinuously changing wavelength is referred to as a vertical mode hopping, and is 0.2 to 0.3 nm/.degree. C.
In order to stabilize the wavelength, a method is generally adopted in which the laser diode is controlled to be maintained at a constant temperature. With this method, it is necessary to attach a temperature control member such as a heater, a radiator or a temperature sensor to the laser diode with a small thermal resistance so as to control the temperature at high accuracy. If this is used for the laser Doppler velocimeter, the apparatus is relatively large and yields high cost, and further the instability due to previously-mentioned vertical mode hopping can not be eliminated completely.
The present applicant has proposed a laser Doppler velocimeter to resolve this problem in European Patent Publication EP 0391278A. According to the method of this Doppler velocimeter (thereinafter referred to as "G-LDV method"), a laser beam from a light source such as a semiconductor laser is incident upon a diffraction grating, among obtained diffracted beams, two diffracted beams at the +n-th and -n-th order (n is 1, 2, . . . ) except for the zeroth order are radiated onto a moving object or flowing fluid at the same intersection angle as an angle made by the two light fluxes, and the scattered light from the moving object or flowing fluid is detected by a photodetector.
FIG. 3 is an example of diffraction when a laser light I is incident upon a transparent diffraction grating 10 having a grating pitch d, in a direction perpendicular to the grating arranging direction t, and the diffraction angle .theta..sub.0 is given by the following expression. EQU sin .theta..sub.0 =m.lambda./d
Where m is a diffraction order (0, 1, 2, . . . ), and .lambda. is a wavelength of the light, in which the lights of the .+-.n-th order except for the zeroth order can be represented by the following expression. EQU sin .theta..sub.0 =.+-.n.lambda./d (3)
(n: 1, 2, . . . )
FIG. 4 is a view in which two light fluxes of the .+-.n-th order are radiated onto a measured object 7 with mirrors 6, 6' so that the incident angle may be .theta..sub.0. The Doppler frequency F of the photodetector 9 is given by: EQU F=2V sin .theta..sub.0 /.lambda.=2nV/d (4)
from the expressions (2) and (3). Therefore, the frequency F is inversely proportional to the grating pitch d of the diffraction grating 10, and proportional to the velocity of the measured object 7, not depending on the laser light I. Since the grating pitch d can be sufficiently stable, the Doppler frequency F can be obtained only in proportion to the velocity of the measured object 7. If the diffraction grating 10 is a reflection type, the same effects can be obtained.
Generally, if a high coherent light such as a laser beam is radiated onto an object, the scattered light due to irregularities of object surface undergoes a random phase modulation, forming spot patterns, i.e., so-called speckle patterns on the observation plane. In the laser Doppler velocimeter, if the moving object is moved, the variation of contrast by the Doppler shift on a detection plane of the photodetector is modulated by irregular variations of contrast due to flow of speckle patterns, and the output signal of the photodetector is also modulated by the variation of transmittance (or reflectance) of measured object.
The G-LDV method as previously described is generally carried out in such a way that the output of the photodetector is passed through a high-pass filter to remove low frequency components electrically in order to pick up only Doppler signals, because the frequency of the variation of contrast due to the flow of speckle patterns and the frequency of the variation of transmittance (or reflectance) for the moving object are generally a low frequency in comparison with the Doppler frequency as shown in the expression (4). But, if the Doppler frequency is low due to a slower velocity of the measured object, the frequency difference with respect to low frequency variation components is smaller, resulting in a problem that the high-pass filter can not be used and the velocity of the measured object can not be measured. Further, the movement direction can not be detected in principle.
Thus, the present applicant has proposed an apparatus having a constitution as shown in FIG. 5 in European Patent Publication EP0391278A. In FIG. 5, the diffraction grating having a grating pitch d is moved at a velocity Vg as shown. A laser beam incident upon the moving diffraction grating is divided into diffracted lights 5a, 5b of the .+-.n-th order, which then undergo positive and negative Doppler shifts .+-.Vg/nd, respectively. The diffraction angle .theta..sub.0 will satisfy: EQU sin .theta..sub.0 =.lambda./nd (5)
(.lambda.: wavelength of the light) If these two light fluxes of the .+-.n-th order are radiated onto the moving object 7 of the velocity V from the mirrors 6, 6' so that the incident angle may be .theta..sub.0, the scattered lights from the measured object 7 undergo the Doppler shift by the amount of +(Vg+V)/nd for the +n-th order light 5 and -(Vg+V)/nd for the -n-th order light 5', and interfere each other, in which the Doppler frequency F is given by: EQU F=2(Vg+V)/nd (6)
As a results, the Doppler frequency F is not dependent upon the wavelength of laser light. That is, even when the velocity of the measured object 7 is slow, the Doppler frequency can be sufficiently taken for a frequency difference with respect to low frequency components caused by the flow of speckle patterns or the variation of transmittance (or reflectance) of the measured object as previously described by the movement velocity Vg of the diffraction grating, and the output signal from the photodetector is passed through the high-pass filter to remove low frequency components electrically in order to pick up only the Doppler signal, whereby the detection of the velocity is enabled.
FIGS. 6A and 6B show the relation between the velocity V of the measured object and the Doppler frequency F in the laser Doppler velocimeter using the diffraction grating. FIG. 6A is an instance where the diffraction grating is fixed, FIG. 6B is an instance where the diffraction grating moves at the velocity Vg. As can be understood from FIG. 6A, even if a certain frequency F1 is detected, the movement direction can not be determined because two velocities V.sub.1, -V.sub.1 of different direction correspond. In FIG. 6B, the Doppler frequency F=Fg+F1 for the velocity V.sub.1 and F=Fg-F1 for the velocity -V.sub.1 are obtained, so that the direction of the velocity V can be detected. From the expression (6), the velocity V is represented by the expression: EQU V=F(d/2)-Vg (7)
Therefore, the velocity V of the expression (7) can be measured by detecting F when the movement velocity Vg of the diffraction grating is controlled.